Cell membrane bilayers have been reconstructed in vitro utilizing total lipid extracts from normal human neural tissue. In agreement with the critical state theory of membrane bilayer assembly (see previous annual report) these lipid extracts spontaneously formed purely unilamellar structures in aqueous dispersion, but only at a critical temperature, T*, equal to the physiological temperature of the tissue; for normal human myelin lipids T* was measured as 37+/-l degree C. The critical state theory has also been applied to two demyelinating diseases to test whether the theory can distinguish among several possible mechanisms for the pathological condition. T* for the total lipid extracts of myelin from a patient with metachromatic leukodystrophy (MLD) was below 30 degrees C, indicating that myelin lipid composition was inappropriate for normal bilayer assembly at the core temperature of this patient. Under these conditions, the critical state theory for bilayer stability predicts that a bilayer will spontaneously degenerate. This mechanism provides a direct link between the metabolic disorder --- a defect in arylsulfatase activity that leads to accumulation of cerebroside sulfate in myelin --- and the formation of pathological myelin. In contrast, total lipid extracts from pathological myelin in a patient with multiple sclerosis, currently believed to be an immune-mediated mechanism and not due to a lipid-disturbance, gave a normal value for T* equal to 37 degrees C. Thus, the critical bilayer theory provides a method for distinguishing when demyelination is caused by a lipid defect. Application of this theory and measurement of T* for detecting other lipid defect-dependent neurological disorders is currently in progress.